Sol-Gel Chemistries

Sol-gel chemistry (Chapter 5) is a preparation method, which can easily be adapted to synthesis robots. The application of this method to high-throughput catalysis was first described by the group of Maier, who prepared amorphous microporous mixed-metal oxides in small cavities of a carrier slate plate [95, 96]. Libraries of doped Ti02, Sn02, and WO3 have been prepared in larger amounts in sets of HPLC flasks [97]. The robot-assisted sol-gel preparation has been applied to mixed-metal oxide catalysts of various composition and the catalysts have been tested for several reactions in gas phases as well as in liquid phase (see Table 11.3). 

Sol-gel chemistry, which consists of the hydrolysis and condensation of typical precursors, is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. The sol-gel method for PDMS modification has some advantages such as high density and homogeneous distribution of particles near the surface [34]. Seki utilized the sol-gel method to bond flexible PDMS and rigid thermoplastics like poly(methyl-methacrylate) (PMMA). 

Inorganic fibers are electrospun from sol-gel systems generally prepared by the hydrolysis of a metal alkoxide [MOR] in acidic alcohol solvents. Although base catalyzed hydrolysis is also possible an acid medium is more likely to yield an electrospinnable solution. A sol in this context refers to a dispersion 

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Inorganic Materials. Sol—gel chemistry involves first the formation of a sol, which is a suspension of soHd particles in a Hquid, then of a gel, which is a diphasic material with a soHd encapsulating a solvent. A detailed description of the fundamental chemistry is available in the Hterature (2—4). The chemistry involving the most commonly used precursors, the alkoxides (M(OR) ), can be described in terms of two classes of reactions ... 

Several different sol-gel chemistries have been described the TPOZ/GTMS chemistry used for aluminum (Section 4.1.3.2), an aromatic silane (m- and p-... 

Methods of Rubber-Silica Hybrid Preparation through Sol-Gel Chemistry.61... 

Sol-gel chemistry offers a unique advantage in the creation of novel organic-inorganic hybrids. The sol-gel process begins with a solution of metal alkoxide precursors [M(0/f) ] and water, where M is a network-forming element, and R is typically an alkyl group. Hydrolysis and... 

METHODS OF RUBBER-SILICA HYBRID PREPARATION THROUGH SOL-GEL CHEMISTRY... 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

Sol-gel chemistry enables us to design hybrid inorganic-organic materials, allowing us to fabricate glasses and films with novel properties and functionalities [24]. The low reac-... 

Sanches, C., Ribot, F. and Lebeau, B. (1999) Molecular design of hybrid organic-inorganic nanocomposites synthesized via sol-gel chemistry. Journal of Materials Chemistry, 9, 35—44. 

Maury, S., Buisson, P., Perrard, A. and Pierre, A.C. (2004) Influence of the sol-gel chemistry on the activity of a lipase encapsulated in a silica aerogel. Journal of Molecular Catalysis B-Enzymatic, 29, 133-148. 

Livage, J., Henry, M. and Sanchez, C. (1988) Sol-gel chemistry of transition-metal oxides. Progress in Solid State Chemistry, 18,... 

Sanchez, C. and Ribot F. (1994) Design of hybrid organic-inorganic materials synthesized via sol-gel chemistry. 

Although sol-gel chemistry is known for decades now, examples of successful market introductions are limited. Furthermore almost all existing sol-gel treatments are solvent-based and consequently difficult to handle. As the global leader in water-based sol-gel chemistry, Evonik offers today a broad variety of commercialized water-based products, which are also available in tons-scale.9... 

Sol—gel, alumina derived from, 23 76—78 Sol-gel bioactive glasses, 23 82-83 Sol-gel chemistry aerogels, 1 749-753 ceramics processing, 5 642 Sol-gel coatings, 5 665... 

Similar organic-inorganic systems, which were ultimately crosslinked by sol-gel chemistry, were prepared with cores composed of high- Tg dendrimers [69, 70]. Tough materials with high heat resistance were obtained. Also, coreshell structures were prepared via silylation, or hyrosilylation. The resulting structures were further crosslinked to give supramolecular assemblies [71]. 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

Characteristic microstructural properties of TiOj membranes produced in this way are given in Table 2.5. Mean pore diameters of 4-5 nm were obtained after heat treatment at T < 500°C. The pore size distribution was narrow in this case and the particle size in the membrane layer was about 5 nm. Anderson et al. (1988) discuss sol/gel chemistry and the formation of nonsupported titania membranes using the colloidal suspension synthesis of the type mentioned above. The particle size in the colloidal dispersion increased with the H/Ti ratio from 80 nm (H /Ti = 0.4, minimum gelling volume) to 140 nm (H /Ti " — 1.0). The membranes, thus prepared, had microstructural characteristics similar to those reported in Table 2.5 and are composed mainly of 20 nm anatase particles. Considerable problems were encountered in membrane synthesis with the polymeric gel route. Anderson et al. (1988) report that clear polymeric sols without precipitates could be produced using initial water concentrations up to 16 mole per mole Ti. Transparent gels could be obtained only when the molar ratio of H2O to Ti is < 4. Gels with up to 12 wt.% T1O2 could be produced provided a low pH is used (H /Ti + < 0.025). 

Strategies for filling the void space of the colloidal crystal utilize sol—gel chemistry, salt precipitation,... 

More complex geometries have been developed [40] and the influence of the geometrical structure has been examined. Although straight-through microchannel emulsification has been developed [39,41], the production rates are still low compared to those obtained with standard emulsification methods. However, the very high monodispersity makes this emulsification process very suitable for some specific fechnological applicafions such as polymeric microsphere synfhesis [42,43], microencapsulation [44], sol-gel chemistry, and electro-optical materials. 

Tethering approaches with Si(OEt)Me2 substituents on the ring, appropriate for both solid state sol-gel chemistry toward Zr-functionalized support and for solution chemistry toward analogous zirconium silsesquioxanes, exist [39]. 

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